Phosphate coating process



3,333,988 PHOSPHATE COATING PROCESS Alfred Douty, Wyncote, Ferdinand P. Heller, Philadelphia, and Lester Steinbrecher, Southampton, Pa., assignors to Amchem Products, Inc., Ambler, Pa., a corporation of Delaware No Drawing. Filed Dec. 16, 1965, Ser. No. 514,415 8 Claims. (Cl. 1486.15)

ABSTRACT OF THE DISCLOSURE Zinc phosphate coating baths accelerated by chlorate have been noted for their instability, and for the rapid and unpredictable deterioration of both the bath and the quality of coatings produced thereby. Chlorate accelerated zinc phosphate coating baths containing about 0.03 to about 1.0 gram/liter nitrite, in accordance with the invention, have no such instability and consistently produce good quality zinc phosphate coatings on ferriferous and zinciferous surfaces.

This invention relates to the art of producing zinc phosphate coatings on metal surfaces by means of chlorate accelerated zinc phosphating solutions. The invention finds its principal application in connection with chlorate accelerated phosphate coating solutions intended for use on ferriferous surfaces such as iron and steel, and zinciferous surfaces such as zinc and galvanized metal, but the invention can also be used with other chlorate accelerated phosphate coating solutions designed for use on other types of metals.

The use of chlorate accelerated zinc phosphate coating solutions for the production of zinc phosphate coatings has been well known for many years. Solutions of this type and their manner of use are disclosed in US. Patent 2,293,716. The present invention is applicable in the use of the coating solutions disclosed in said patent and the disclosures and teachings of the patent are incorporated herein by reference. The invention is also applicable to various modifications of the chlorate accelerated phosphate coating solutions disclosed in Patent 2,293,716 which have been developed and are conventionally used. Among these are chlorate accelerated solutions containing nickel compounds which have been found to be more suitable for use on galvanized metal.

The application of a zinc phosphate coating on steel or galvanized steel produces a material which has increased resistance to corrosion and one to which decorative coatings such as paint and lacquer adhere better. Moreover, zinc phosphate coatings on steel, when combined with lubricants, facilitate cold forming of the metal.

Chlorate accelerated zinc phosphating baths are capable of producing exceptionally fine grained coatings on the metal. Such coatings are particularly useful and desirable when the metal is to be given a final finish of high gloss. Unfortunately, zinc phosphate solutions accelerated with chlorate are notoriously unpredictable. The solutions generally produce good coatings on the metal when they are freshly prepared, and sometimes will continue to produce good coatings for a long time. However, quite often the quality of the coating produced deteriorates after the bath has been used only a short time. No way has been devised to obtain a prediction of the time when a bath will become unusable. Nor has it been possible heretofore to pinpoint the causes of deterioration. Thus it has often happened that a bath will become unusable after only a few hours even though the control parameters, such as pH, free to total acid ratio, and zinc content, have been carefully observed and maintained within normal limits. In other words, to the nited States Patent 3,333,988 Patented Aug. 1, 1967 operator of the bath, an unusable bath, or one on the verge of becoming unusable, appears to differ in no material respect from a fresh bath.

It can be easily appreciated that considerable economic loss occurs if a large bath of several thousand gallons becomes unusable after only a few hours and must be discarded. The unpredictable quality of chlorate accelerated phosphate baths appears to be aggravated when the bath is used in a spray application system operated at low temperatures such as l35-100 F., although the unpredictability is not eliminated by operating at a higher temperature.

In an attempt to ameliorate this condition it has been suggested that an arsenate be added to the bath. However, such materials are toxic and thus have obvious disadvantages both from an operations standpoint and from a product standpoint.

The principal object of the present invention is to provide a method for maintaining chlorate accelerated zinc phosphate coating solutions of the kind known in the art in good condition so that they can be used with customary replenishment for long periods of time without encountering the difficulties resulting from deterioration of the solution as discussed above.

Another object of the invention is to provide a new zinc phosphatizing process which is chlorate accelerated, and which is especially suited for use in a spray operation on a continuous or semi-continuous basis for long periods of time without deterioration in the quality of work produced by the process.

A related object of the invention is the provision of improved chlorate accelerated zinc phosphating solutions which may be reliably operated for long periods of time.

Still another object of the invention is the provision of a chlorate accelerated zinc phosphatizing process which can be reliably operated at lower temperatures than those which have heretofore been found necessary for production scale operation. In this connection it is pointed out that when chlorate accelerated zinc phosphating solutions were in the early stages of development, it was thought that the temperature of operation presented no serious problem and that the solutions could be operated successfully over a wide range of temperatures above about F. See the discussion in the above mentioned Patent 2,293,716. However, as production experience was gained in the use of the chlorate accelerated process, it was found that temperature was an important factor and that the process as a practical matter had to be operated at temperatures above about F. Such high operating temperatures increase the cost of the process and present control and other operating problems which reduce the desirability of the process.

Several discoveries underlie the present invention. The first of these is the discovery that small amounts of nitrite ion are relatively stable in the presence of chlorate ion in chlorate accelerated zinc phosphating solutions. This discovery was unexpected, because chlorate ion is known to be a strong oxidizing agent and would be expected to oxidize the nitrite to nitrate quite quickly under the conditions obtaining in a zinc phosphate solution. The inven-- tion is also based on the discovery that when nitrite ion is incorporated in, and maintained in, chlorate accelerated zinc phosphating baths in concentrations discussed below, the bath will produce zinc phosphate coatings of the highest quality on ferriferous and zinciferous surfaces for long periods of time. The nitrite containing chlorate accelerated solutions are free from the danger of unpredictable deterioration in quality which has limited the utility of chlorate accelerated solutions of the prior art.

The reasons for the radical improvement obtained by including nitrite ion in chlorate accelerated zinc phosphating solutions are not fully understood, and there is no intention to limit the scope of the present invention to a particular theory of the mechanism by which the improvement is obtained. However, we have made some further discoveries which we think furnish the basis for postulating the mechanisms by which nitrite inclusion in the solutions yields superior results. We have discovered that ferrous iron can exist in a chlorate accelerated zinc phosphating bath. This was unexpected since the bath contains chlorate, a strong oxidizing agent. The chlorate would be expected to oxidize the ferrous iron to ferric iron. But the oxidizing reaction appears to be so slow that an appreciable buildup of ferrous ion can occur during the course of coating operations. The source of the ferrous ion is the surface being coated. It appears that the unexpected buildup of ferrous ion in the chlorate accelerated phosphating baths is at least one of the causes of their unpredictable deterioration. We have discovered that nitrite ion in a chlorate accelerated bath oxidizes ferrous iron to ferric iron much more rapidly than does chlorate, and oxidizes the iron despite the presence of the chlorate. It is believed that the effective removal of ferrous ion achieved by the nitrite is the mechanism by which the improved performance of the chlorate accelerated baths is obtained. However, it should be kept in mind that the nitrite may possibly perform other functions contributing to the improved result.

We have also discovered that When nitrite ion is utilized in a chlorate accelerated phosphating solution the solution can be used for coating at considerably lower temperatures than those found necessary in the past. The solutions will produce good coatings at temperatures as low as 100 F.

Apart from the nitrite component, the improved coating solutions of the invention have two principal coating producing constituents, zinc acid phosphate, and chlorate. As was mentioned above, other components have been included in chlorate accelerated solutions in the past for various purposes. Thus nitrate has sometimes been included as a component, although the improvement in result gained from its use is marginal and difficult to isolate. In fact, it is arguable that no significant improvement is gained at all. However, some prefer, nonetheless, to include nitrate in the coating bath. Copper is sometimes included as an element of the bath because it is thought that it facilitates coating of hard surfaced metal and reduces the time required for formation of the coating. Nickel has been used as a component of the coatings solution because it improves the coating action on zinciferous surfaces. A chlorate accelerated bath containing nickel is actually more versatile than one containing only the basic components, because it can be used to coat both galvanized steel and steel. Various other addition agents have been used for special purposes.

The present invention is fully applicable to substantially all of the forms of chlorate accelerated zinc phosphating baths heretofore employed by the prior art, and yields a material improvement in the performance of such baths. The improved results are obtained notwithstanding variations of the nature and quantity of the components of the bath within the limits contemplated by the prior art.

In accordance with the invention there is included in the chlorate accelerated zinc phosphating solutions from about 0.03 to about 1.0 gram/ liter of nitrite ion expressed as N The quantity of nitrite to be included is somewhat afi'ected by the temperature at which the coating operation is performed. At low temperatures such as 100-120 F. we have found that as litle as 0.03 gram/ liter of nitrite will bring about the desired results. As the operating temperature rises, the amount of nitrite which yields optimum results also rises. Thus, at temperatures of from about 120 to about 140 F. and higher, we prefer to use at least 0.07 gram/liter of nitrite ion. The upper limits of concentration of nitrite have been found to be less critical than the lower limits. Thus, coating baths containing as much as 1.0 gram/liter nitrite ion at 140 F. are fully operative and yield the improved results of the invention. Economic considerations give rise to a preferred upper limit of concentration somewhat lower than this value, since sigh concentrations of nitrite do not yield improvements in result which are commensurate with the added cost of establishing and maintaining such higher concentrations. Thus we prefer to utilize for optimum economy between about 0.07 and 0.11 gram/liter of nitrite calculated as sodium nitrite. When the nitrite concentration is maintained within this range, assurance is obtained that enough nitrite is present to yield the superior results of the invention without undue waste of the nitrite.

The methods and solutions of the present invention can be understood from the following illustrations. In the first of these a concentrate for use in a chlorate accelerated zinc phosphating process was prepared according to the following formula.

F ormtula #1 Chemicals used: Percent by weight Zinc oxide 10.73 Phosphoric .acid (75%) 47.36 Sodium chlorate 5.83

Water, to make 100.00

The concentrated solution was conventionally prepared by dissolving the zinc oxide in the phosphoric acid, adding the water to cool the solution to about F., and finally adding the sodium chlorate.

The concentrate of Formula #1 was used both as a component of the coating bath and as a replenishing material for addition to the bath during the course of operation. The coating bath was prepared by dissolving 3040 grams of the concentrate in 100 liters of water. In addition, 180 grams of sodium chlorate were added to the bath at the outset. The additional sodium chlorate may be throught of as an optional ingredient, since an operative bath can be made from the concentrate alone. However, if the additional quantity of sodium chlorate is not used, the bath is not as efiicient in operation at the outset. Furthermore, the additional 180 grams of chlorate adjust the zinc phosphate/chlorate ratio so that the concentrate of Formula #1 is a very satisfactory replenishing material when the bath is used in a spray process.

In accordance with the invention, there was added to the bath at the outset 12 grams of sodium nitrite. The pH of an efiicient chlorate accelerated bath should be from about 2.5 to 3.5 and the bath formed in the manner just described was adjusted to meet this requirement by the addition of sodium hydroxide.

The coating bath was heated to F. and was used to coat clean strip panels of steel in a spray system utilizing a one minute spray contact for coating purposes.

As the coating operation proceeded, sufficient quantities of Formula #1 were added to maintain the zinc content and free to total acid ratio within normal limits taught by the prior art. The pH required no further adjustment. In addition, sodium nitrite was added during the operation in quantities suflicient to maintain the concentration thereof from about 0.07 to about 1.0 gram/liter as N0 The coatings produced by the bath were of excellent quality and the bath showed no signs of deterioration even after it had been run for many hours.

An example of the application of the invention to coating operation may be seen from the following illustration. A second chlorate accelerated zinc phosphate concentrate was prepared according to the following formula.

Formula #2 Chemicals used: Percent by weight This. concentrate was prepared in a manner similar to that of Formula #1, by dissolving the zinc oxide and the nickelous oxide in the phosphoric acid, adding the water to cool the solution to 100 F. and finally adding the sodium chlorate. Nickel was used in this concentrate in order to increase the versatility of the coating bath by making it suitable for coating zinc as well as ferrous surfaces. See Romig U.S. Patent 2,121,574.

A coating bath was prepared from the concentrate of Formula #2 by dissolving 3040 grams of-the concentrate in 100 liters of water. Again, 180 grams of sodium chlorate were added to the bath at the outset. The reasons for this addition are the same as the reasons given above in connection with the similar addition of sodium chlorate in the previous illustration. Twelve grams of sodium nitrite were added to the bath made up from Formula #2, and sutiieient sodium hydroxide was added to adjust the pH to approximately 3.2.

This bath was heated to 140' F. and was used to coat clean strip panels of steel and galvanized steel in a spray operation using a one minute contact time of the metal with the spray. After 220 square feet of metal were treated it was found that 2.64 pounds of the concentrate of Formula #2 per 1000 square feet of metal treated was required as normal replenishment to keep the bath in proper operating condition. it was also found that 0.27 pound per 1000 square feet of metal treated of sodium nitrite was consumed and had to be added to maintain the nitrite concentration between 0.07 and 1.0 gram/liter of bath.

The coating on the steel weighed approximately 150 milligrams per square foot and was exceptionally fine grained.

After treating the quantity of metal described above, the bath temperature was allowed to drop to about 130' F. and the run was continued. in this portion of the run, 183 square feet of steel and 132 square feet of galvanized steel were treated with excellent results. Replenishment by means of Formula #2 and sodium nitrite was carried out in accordance with the proportions outlined for the first part of the run.

The run was continued into a third phase. During this phase steel was treated by the bath and replenishing material of Formula #2 was added to maintain the control parameters within normal limits. However, replenishing additions of nitrite were discontinued and the nitrite level was allowed to drop. The decrease in nitrite level was monitored. As the concentration of nitrite in the bath fell oil, there was observed a deterioration of the quality of the coatings produced. By the time that the nitrite had completely disappeared from the bath, the coatings were streaky and rusty and completely unsuitable as a paint base.

The run was continued into a fourth phase. During this phase nitrite was again added to the bath to establish and maintain a concentration above about 0.07 grarn/ liter. The addition of nitrite restored the coating bath to good operating condition and the quality of the coatings produced by the bath on steel became excellent once again.

It should be noted that during the second, third, and fourth phases of the run described above, the bath temperature was maintained at about 130' R, which is considerabiy cooler than has heretofore been found practical in production operations. it should also be noted that although the sodium nitrite was employed in both illustrations as a source of nitrite, other soluble sources of nitrite can be used, including both salts and nitrous acid.

Control of the process of the invention is easily accomplished. The control parameters which have been used by the art, namely, zinc concentration, free to total acid ratio, and pH, are suitable for regulating the addition of replenishing concentrate for maintaining the zinc and chlorate level in the treating bath. Control of the nitrite concentration may be accomplished in several ways. For example, nitrite level can be monitored and used for control purposes in accordance with U.S. application Ser. No. 390,375. Alternatcly, samples can be placed in a standard pH meter and titrated with ferrous ammonium sulfate to an inflection point. Still another means of measuring nirrite concentration is by a standard colorimetric method involving the formation of an azo dye. In this connection see APHAStandard Methods for Examination of Water and Waste Water (1960), pages 303-304.

We claim:

1. A method for maintaining the coating producing quality of a chlorate accelerated zinc phosphating coating solution of the type used to apply a phosphate coating to surfaces from the group consisting of ferriferous and zinciferous surfaces comprising adding chlorate and zinc acid phosphate to said coating solution in amounts sutiicient to replenish the depletion of chlorate and zinc acid phosphate occuring during the course of use of said solution, and adding nitrite to said coating solution in amounts sufiicient to establish and maintain a concentration of nitrite therein of at least 0.03 gram/liter calculated as NO, during the course of use of said solution.

2. A method in accordance with claim 1 in which the temperature of the coating solution is maintained at from about F. to about F. during the course of use of said solution.

3. A method in accordance with claim 1 in which the pH of the coating solution is maintained at from about 2.5 to about 3.5 during the course of use of said solution.

4. A method in accordance with claim 3 in which the concentration of nitrite is established and maintained at a level of from about 0.07 to about 0.11 gram/liter during the course of use of said solution.

5. A method in accordance with claim 4 in which the temperature of the coating solution is maintained at from about 120' F. to about F. during the course of use of said solution.

6. A process for applying a zinc phosphate coating to surfaces from the group consisting of ferrit'erous and zint'iferous surfaces comprising contacting said surfaces with a coating solution containing as essential coating producing ingredients rinc acid phosphate, chlorate and from about 0.03 to about 1.0 gram liter nitrite calculated as NO,, said solution having a pH of from about 2.5 to about 3.5, replenishing said nitrite as it is consumed during coating by adding additional nitrite to maintain the concentration thereof, and maintaining the temperature of said solution above 100' F. during coating therewith.

7. An aqueous solution for applying a zinc phosphate coating to metal surfaces comprising as essential coating producing ingredients zinc acid phosphate, chlorate, and from about 0.03 to about 1.0 gram/liter nitrite calculated 83 N0 8. A solution in accordance with claim 7 and further comprising nickel ion.

References Cited UNITED STATES PATENTS 2,790,740 4/1957 Ayres 148-6.]5 2,987,428 6/1961 Heller 148-615 3,129,123 4/1964 Rodzewich 1486.15 3,144,361 8/1964 Klinghofl'er 148-615 X ALFRED L. LEAVIT'I, Primary Examiner.

R. S. KENDALL, Assistant Examiner. 

1. A METHOD FOR MAINTAINING THE COATING PRODUCING QUALITY OF A CHLORATE ACCELERATED ZINC PHOSPHATING COATING SOLUTION OF THE TYPE USED TO APPLY A PHOSPHATE COATING TO SURFACES FROM THE GROUP CONSISTING OF FERRIFEROUS AND ZINCIFEROUS SURFACES COMPRISING ADDING CHLORATE AND ZINC ACID PHOSPHATE TO SAID COATING SOLUTION IN AMOUNTS SUFFICIENT TO REPLENISH THE DEPLETION OF CHLORATE AND ZINC ACID PHOSPHATE OCCURING DURING THE COURSE OF USE OF SAID SOLUTION, AND ADDING NITRITE TO SAID COATING SOLUTION IN AMOUNTS SUFFICIENT TO ESTABLISH AND MAINTAIN A CONCENTRATION OF NITRITE THEREIN OF AT LEAST 0.03 GRAM/LITER CALCULATED AS NO2 DURING THE COURSE OF USE OF SAID SOLUTION. 